Method of producing cold-rolled steel sheet as well as cold-rolled steel sheet and members for automobile

ABSTRACT

In a method of producing a cold-rolled steel sheet being excellent in not only the phosphate treatability but also the corrosion resistance after coating under severe corrosion environment such as hot salt water immersion test or composite cycle corrosion test, a continuously annealed steel sheet after cold rolling preferably including 0.5-3.0 mass % of Si is pickled to remove a Si-containing oxide layer on a surface layer of the steel sheet and further repickled so that a surface covering ratio of an iron-based oxide on the surface of the steel sheet is not more than 40% and preferably a maximum thickness of the iron-based oxide is not more than 150 nm, as well as a cold-rolled steel sheet produced by this method and a member for automobile using the cold-rolled steel sheet.

TECHNICAL FIELD

This invention relates to a method of producing a cold-rolled steelsheet as well as a cold-rolled steel sheet and a member for automobile,and more particularly to a method of producing a cold-rolled steel sheetbeing excellent in not only the phosphate treatability but also thecorrosion resistance after coating as evaluated by a hot salt waterimmersion test or a composite cycle corrosion test, a cold-rolled steelsheet produced by this method as well as a member for automobile usingthe cold-rolled steel sheet.

Moreover, the cold-rolled steel sheet according to the invention can bepreferably used in a high-strength cold-rolled steel sheet containing Siand having a tensile strength TS of not less than 590 MPa.

BACKGROUND ART

Recently, it is strongly demanded to improve fuel consumption of anautomobile from a viewpoint of the protection of global environment.Also, it is strongly demanded to improve the safety of the automobilefrom a viewpoint of ensuring the safe of crew members at the time ofimpact. In order to meet these demands, it is required to simultaneouslyattain weight reduction and high-strengthening of a vehicle body in theautomobile, while the thinning associated with the high strengthening ispositively proceeding in cold-rolled steel sheets as a starting materialin the member for automobile. However, many members for automobile aremanufactured by forming the steel sheet, so that these steel sheets arerequired to have an excellent formability in addition to the highstrength.

There are various methods for enhancing the strength of the cold-rolledsteel sheet. As a method increasing the strength without largelydamaging the formability is mentioned a solid-solution strengtheningmethod through addition of Si. However, when a greater amount of Si,particularly not less than 0.5 mass % of Si is added to a cold-rolledsteel sheet, it is known that Si-containing oxides such as SiO₂, Si—Mnbased composite oxide and the like are formed on the surface of thesteel sheet during slab heating or during annealing after hot rolling orafter cold rolling. Since the Si-containing oxide considerablydeteriorates the phosphate treatability, the high-strength cold-rolledsteel sheets containing a great amount of Si have problems that thephosphate treatability is poor but the coating peeling is easily causedto deteriorate the corrosion resistance after the coating as comparedwith the commonly used steel sheets when the steels sheet afterelectrodeposition coating is subjected to severer corrosion environmentas in a hot salt water immersion test or a composite cycle corrosiontest repeating cycle of wetting-drying.

As a countermeasure for these problems, for example, Patent Document 1proposes a high-strength cold-rolled steel sheet obtained by heating aslab at a temperature of higher than 1200° C. in hot rolling, descalingunder high pressure, polishing the surface of the hot-rolled steel sheetwith a nylon brush containing abrasion grains prior to pickling and thenimmersing in a bath of 9% hydrochloric acid twice to perform pickling tolower the Si concentration on the surface of the steel sheet. Also,Patent Document 2 proposes a high-strength cold-rolled steel sheetwherein the corrosion resistance is improved by rendering line width ofSi-containing linear oxide observed in 1-10 pm from the surface of thesteel sheet into not more than 300 nm.

However, in the high-strength cold-rolled steel sheet disclosed inPatent Document 1, even if the Si concentration on the surface of thesteel sheet is reduced before the cold rolling, the Si-containing oxideis formed on the surface of the steel sheet by annealing after coldrolling, so that the improvement of the corrosion resistance aftercoating is not desired. Also, in the high-strength cold-rolled steelsheet disclosed in Patent Document 2, there is no problem in thecorrosion resistance under corrosion environment as in a salt spray testdefined according to JIS Z2371, but sufficient corrosion resistanceafter coating is not obtained under severer corrosion environment as ina hot salt water immersion test or a composite cycle corrosion test.That is, the high-strength cold-rolled steel sheet having an excellentcorrosion resistance after coating can not be obtained only by reducingthe Si concentration on the surface of the steel sheet after hot rollingor by reducing the Si-containing linear oxide.

As a technique for solving the above problems, Patent Document 3discloses a technique wherein the Si-containing oxide enriched on thesurface of the steel sheet by annealing step or the like is removed bypickling and further an S-based compound is applied to the surface toenhance the reactivity with a phosphate treating solution to therebyimprove the phosphate treatability. Also, Patent Document 4 discloses atechnique wherein a P-based compound is applied instead of the S-basedcompound of the above technique.

[Prior Art Articles] [Patent Document]

[Patent Document 1] JP-A-2004-204350

[Patent Document 2] JP-A-2004-244698

[Patent Document 3] JP-A-2007-217743

[Patent Document 4] JP-A-2007-246951

SUMMARY OF THE INVENTION [Problems to be Solved by the Invention]

In recent years, for the purpose of reducing industrial wastes(suppression of sludge formation) and cutting down running cost, it isproceeded to lower the temperature of the phosphate treating solution,and hence the reactivity of the phosphate treating solution to the steelsheet is largely lowered as compared with the conventional phosphatetreating conditions. The lowering of the temperature of the treatingsolution does not come into problem when the surface adjusting techniqueprior to the phosphate treatment is improved in the common steel sheethaving a less addition amount of alloy usually used. However, in thehigh-strength cold-rolled steel sheet added with a great amount of Si,the reactivity with the phosphate treating solution is considerablydeteriorated by the influence of the Si-containing oxide formed on thesurface of the steel sheet at an annealing step, so that it is requiredto enhance the reactivity from the steel sheet side in some way. On theother hand, the techniques disclosed in Patent Documents 3 and 4 areeffective to the conventional common steel sheets, but can not expectthe sufficient improving effect capable of lowering the temperature ofthe phosphate treating solution for the high-strength cold-rolled steelsheets containing a great amount of Si.

The invention is made in view of considering the above problems inherentto the cold-rolled steel sheet containing a great amount of Si and is toprovide a method of producing a cold-rolled steel sheet being excellentin not only the phosphate treatability even when using a phosphatetreating solution at a lower temperature but also in the corrosionresistance after coating as evaluated by a hot salt water immersion testor a composite cycle corrosion test, a cold-rolled steel sheet producedby this method as well as a member for automobile using the cold-rolledsteel sheet.

[Means For Solving Problems]

The inventors have made detailed analysis on surface properties of steelsheets after annealing in order to solve the above problems and variousstudies on a method of enhancing the reactivity between the surface ofthe steel sheet and the phosphate treating solution. As a result, it hasbeen found that it is very important to subject the continuouslyannealed steel sheet surface to strong pickling after the cold rollingto thereby remove Si-containing oxide layer formed on the surface of thesteel sheet during the annealing but also reduce a ratio of covering thesurface of the steel sheet with an iron-based oxide formed on the steelsheet surface by the strong pickling, and consequently the invention hasbeen accomplished.

That is, the invention proposes a method of producing a cold-rolledsteel sheet, comprising steps of cold rolling a steel sheet,continuously annealing, pickling and further repickling it.

The repickling in the production method of the invention ischaracterized in that a non-oxidizable acid is used instead of an acidused in the pickling prior to the repickling.

The non-oxidizable acid in the production method of the invention ischaracterized to be any of hydrochloric acid, sulfuric acid, phosphoricacid, pyrophosphoric acid, formic acid, acetic acid, citric acid,hydrofluoric acid, oxalic acid and a mixed acid of two or more thereof.

The non-oxidizable acid in the production method of the invention ischaracterized to be any of hydrochloric acid with a concentration of0.1-50 g/L, sulfuric acid with a concentration of 0.1-150 g/L and amixed acid of 0.1-20 g/L of hydrochloric acid and 0.1-60 g/L of sulfuricacid.

Also, the production method of the invention is characterized in thatthe repickling is carried out at a temperature of a repickling solutionof 20-70° C. for 1-30 seconds.

Furthermore, the production method of the invention is characterized inthat the pickling is carried out with any of nitric acid, hydrochloricacid, hydrofluoric acid, sulfuric acid and a mixed acid of two or morethereof.

Moreover, the production method of the invention is characterized inthat the pickling is carried out with any of a mixed acid of nitric acidand hydrochloric acid wherein a concentration of nitric acid is morethan 50 g/L but not more than 200 g/L and a ratio (HCl/HNO₃) ofhydrochloric acid concentration to nitric acid concentration is0.01-1.0, or a mixed acid of nitric acid and hydrofluoric acid wherein aconcentration of nitric acid is more than 50 g/L but not more than 200g/L and a ratio (HF/HNO₃) of hydrofluoric acid concentration to nitricacid concentration is 0.01-1.0.

The steel sheet in the production method of the invention ischaracterized by comprising 0.5-3.0 mass % of Si.

Also, the steel sheet in the production method of the invention ischaracterized by having a chemical composition comprising, in additionto Si, C: 0.01-0.30 mass %, Mn: 1.0-7.5 mass %, P: not more than 0.05mass %, S: not more than 0.01 mass %, Al: not more than 0.06 mass % andthe remainder being Fe and inevitable impurities.

Furthermore, the steel sheet in the production method of the inventionis characterized by containing, in addition to the above chemicalcomposition, one or more selected from Nb: not more than 0.3 mass %, Ti:not more than 0.3 mass %, V: not more than 0.3 mass %, Mo: not more than0.3 mass %, Cr: not more than 0.5 mass %, B: not more than 0.006 mass %and N: not more than 0.008 mass %.

Moreover, the steel sheet in the production method of the invention ischaracterized by containing, in addition to the aforementioned chemicalcomposition, one or more selected from Ni: not more than 2.0 mass %, Cu:not more than 2.0 mass %, Ca: not more than 0.1 mass % and REM: not morethan 0.1 mass %.

The invention is a cold-rolled steel sheet produced by any one of theaforementioned methods, characterized in that a Si-containing oxidelayer is removed from a surface layer of the steel sheet by picklingafter continuous annealing and a surface covering ratio of an iron-basedoxide existing on the surface of the steel sheet after repickling is notmore than 40%.

Also, the cold-rolled steel sheet of the invention is characterized inthat a maximum thickness of the iron-based oxide existing on the steelsheet surface after repickling is not more than 150 nm.

Further, the invention is a member for automobiles, characterized byusing a cold-rolled steel sheet as described in any one of the above.

[Effect Of The Invention]

According to the invention, there can be provided a cold-rolled steelsheet which is excellent in the phosphate treatability even when Si iscontained as large as 0.5-3.0 mass % and when using a phosphate treatingsolution at a lower temperature but also is excellent in the corrosionresistance after coating under severer corrosion environment as in a hotsalt water immersion test or a composite cycle corrosion test. Accordingto the invention, therefore, it is possible to largely improve thephosphate treatability and corrosion resistance after coating in thehigh-strength cold-rolled steel sheets containing a greater amount of Siand having a tensile strength TS of not less than 590 MPa, so that itcan be preferably used in strong members and the like in a vehicle bodyof an automobile.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows reflection electron microphotographs of steel sheetsurfaces of standard cold-rolled steel sheet sample Nos. a and b fordetermining a surface covering ratio with an iron-based oxide.

FIG. 2 shows a histogram of pixel number to gray value in the reflectionelectron microphotographs of the standard cold-rolled steel sheet sampleNos. a and b.

FIG. 3 is a photograph of a section of a coating on a surface of a steelsheet after repickling observed by means of a transmission electronmicroscope.

FIG. 4 is a graph showing energy dispersion type X-ray (EDX) analyticalresults of an iron-based oxide observed in FIG. 3.

FIG. 5 is a graph of depth distribution of O, Si, Mn and Fe on a surfaceof a test specimen in Comparative Example (No. 1) and Invention Example(No. 9) of Example 1 as measured by GDS.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

First, the basic technical idea of the invention will be described.

In an annealing step using a continuous annealing furnace forrecrystallizing a cold-rolled steel sheet after cold rolling to impartdesired structure, strength and workability, a non-oxidizing or reducinggas is usually used as an atmosphere gas, and also a dew point isstrictly controlled. In the commonly general cold-rolled steel sheethaving a less amount of an alloy added, therefore, the oxidation of thesteel sheet surface is controlled. However, in the steel sheetcontaining not less than 0.5 mass % of Si or Mn, even if component ordew point of the atmosphere gas in the annealing is strictly controlled,it can not be avoided that Si, Mn and the like being easily oxidizableas compared with Fe are oxidized to form a Si-containing oxide such asSi oxide (SiO₂), Si—Mn based composite oxide or the like on the surfaceof the steel sheet. The construction of these oxides varies depending oncomponents of the steel sheet, annealing atmosphere and the like, butboth the oxides are typically and frequently existent in a mixture.Also, since the Si-containing oxide is formed not only the surface ofthe steel sheet but also in the interior of the steel matrix, it isknown that the oxide obstructs the etching property on the surface ofthe steel sheet in the phosphate treatment (treatment with zincphosphate) made as an underlaying treatment for electrodepositioncoating and badly affects the formation of sound phosphate treatedcoating.

In recent years, the lowering of the temperature of the phosphatetreating solution is proceeding for the purpose of reducing the sludgeamount generated in the phosphate treatment and the running cost, andhence the phosphate treatment is carried out under a condition that thereactivity of the phosphate treating solution to the steel sheet isconsiderably low as compared with the conventional technique. The changeof the phosphate treating condition is not particularly questioned bythe improvement of the surface adjusting technique or the like in theconventionally used common steel sheets having a less addition amount ofan alloy. In the steel sheet having a greater addition amount ofalloying component, particularly a high-strength cold-rolled steel sheetattempted to increase the strength by adding a greater amount of Si,however, the influence of changing the phosphate treating condition asmentioned above is very large. In the cold-rolled steel sheet having agreater amount of Si, therefore, it is required that the surface of thesteel sheet itself is activated in correspondence with the deteriorationof the phosphate treating condition to enhance the reactivity with thephosphate treating solution.

The inventors have made various investigations on a method of improvingthe phosphate treatability for corresponding to the deterioration of thephosphate treating condition as mentioned above. As a result, it hasbeen found out that it is effective to conduct strong pickling of thesurface of the cold-rolled steel sheet after continuous annealing withnitric acid or the like as a pickling solution to remove a Si-containingoxide layer formed on the surface of the steel sheet by continuousannealing and the like after cold rolling. The term “Si-containingoxide” used herein means SiO₂ or Si—Mn base composite oxide formed onthe surface of the steel sheet or along crystal grain boundary insidethe steel sheet in the slab heating or after hot rolling or in annealingafter cold rolling. The thickness of the layer containing theseSi-containing oxides varied depending upon components of the steel sheetor the annealing condition (temperature, time, atmosphere), but isusually about 1 μm from the surface of the steel sheet. Also, the term“removal of the Si-containing oxide layer” according to the inventionmeans that the pickling is carried out to remove the Si-containing oxidelayer to a level that peaks of Si, O do not appear when the surface ofthe steel sheet is analyzed in depth direction by means of GDS (glowdischarge optical emission spectroscopy).

The reason why a strong acid such as nitric acid or the like is used asthe pickling solution is due to the fact that among the Si-containingoxides, Si—Mn based composite oxide is easily dissolved in an acid, butSiO₂ is hardly soluble, and in order to remove the latter, theSi-containing oxide on the surface of the steel sheet should be removedtogether with the steel matrix.

According to the inventors' studies, however, it can be seen that thephosphate treatability is largely improved by removing the Si-containingoxide layer existing on the steel sheet surface through strong picklingwith nitric acid or the like after the continuous annealing but thephosphate treatability may be deteriorated at moments. As the cause isfurther investigated, it is newly found that although the Si-containingoxide layer is removed by the strong pickling with nitric acid or thelike, Fe dissolved from the surface of the steel sheet by the picklingseparately produces an iron-based oxide, which is settled andprecipitated on the surface of the steel sheet so as to cover the steelsheet surface to thereby deteriorate the phosphate treatability.

And, it has been found that in order to suppress the oxidation of thesteel sheet surface by the above strong pickling to mitigate the badinfluence upon the phosphate treatability, it is important to suppressthe formation of the iron-based oxide on the steel sheet surface toreduce the ratio of covering the steel sheet surface with the iron-basedoxide to not more than 40% and that it is effective as means forattaining the above to further conduct repickling under adequateconditions after the pickling to dissolve and remove the iron-basedoxide precipitated on the surface of the steel sheet.

Further, the inventors have found that the phosphate treatability ismore improved and the corrosion resistance is further improved when themaximum thickness of the iron-based oxide is not more than 150 nm inaddition to the fact that the covering ratio of the iron-based oxidegenerated on the surface of the steel sheet by pickling is not more than40% and that it is effective as means for attaining the above to conductthe repickling by properly increasing the concentration of the acid usedin the repickling.

Moreover, the iron-based oxide in the invention means an oxide composedmainly of iron wherein an atomic concentration ratio of iron is not lessthan 30% as an element other than oxygen constituting the oxide. Theiron-based oxide is existent on the surface of the steel sheet at anuneven thickness, which is different from a natural oxide film existinguniformly and in layer at a thickness of few nm. The iron-based oxidegenerated on the surface of the cold-rolled steel sheet is confirmed tobe amorphous from the observation by means of a transmission electronmicroscope (TEM) and analysis results of diffraction pattern (analyticaldiagram) through an electron diffractometry.

The invention is accomplished by conducting further examinations on theabove new knowledge.

The reason why the chemical composition of the cold-rolled steel sheetaccording to the invention is limited to the above range will bedescribed below.

Si: 0.5-3.0 mass %

Si is an element effective for attaining the increase of the strength ofthe steel because the effect of enhancing the strength of steel(solid-solution strengthening ability) is large without largely damagingthe workability, but is also an element adversely exerting on thephosphate treatability and the corrosion resistance after coating. WhenSi is added as means for attaining a high strength, the addition of notless than 0.5 mass % is necessary. If the Si content is less than 0.5mass %, the influence due to the deterioration of the phosphate treatingconditions is less. On the other hand, when the Si content exceeds 3.0mass %, the hot rolling property and cold rolling property are largelydeteriorated, which is adversely influenced on the productivity andleads to the deterioration of ductility of the steel sheet itself.Therefore, Si is added within a range of 0.5-3.0 mass %. Preferably, itis a range of 0.8-2.5 mass %.

The cold-rolled steel sheet of the invention is an essential feature toinclude Si in the above range. The other components are acceptable asfar as they are included within composition ranges in the commoncold-rolled steel sheet, and are not particularly limited. However, thecold-rolled steel sheet of the invention is preferable to have thefollowing component composition when it is applied to a high-strengthcold-rolled steel sheet having a tensile strength of not less than 590MPa for use in vehicle bodies for automobiles and so on.

C: 0.01-0.30 mass %

C is an element effective for enhancing the strength of steel andfurther is an element effective for producing residual austenite havingan effect of TRIP (Transformation Induced Plasticity), bainite andmartensite. When C content is not less than 0.01 mass %, the aboveeffect is obtained, while when C content is not more than 0.30 mass %,the deterioration of the weldability is not caused. Therefore, C isadded preferably within a range of 0.01-0.3 mass %, more preferablywithin a range of 0.10-0.20 mass %.

Mn: 1.0-7.5 mass %

Mn is an element having an action for solid-solution strengthening steelto increase the strength and enhance the hardenability and promoting theformation of residual austenite, bainite and martensite. Such effectsare developed by the addition of not less than 1.0 mass %. On the otherhand, when Mn content is not more than 7.5 mass %, the above effect isobtained without the increase of the cost. Therefore, Mn is addedpreferably within a range of 1.0-7.5 mass %, more preferably within arange of 2.0-5.0 mass %.

P: not more than 0.05 mass %

P is an element damaging no drawability though the solid-solutionstrengthening ability is large and is also an element effective forattaining a high strength, so that it is preferable to be included in anamount of not less than 0.005 mass %. However, P is an element damagingthe spot weldability, but there is no problem when it is not more than0.05 mass %. Therefore, P is preferably not more than 0.05 mass %, morepreferably not more than 0.02 mass %.

S: not more than 0.01 mass %

S is an impurity element inevitably incorporated, and is a harmfulelement which is precipitated in steel as MnS to deteriorate thestretch-flanging property. In order to prevent the deterioration of thestretch-flanging property, S is preferably not more than 0.01 mass %,more preferably not more than 0.005 mass %, further preferably not morethan 0.003 mass %.

Al: not more than 0.06 mass %

Al is an element added as a deoxidizer at steel-making step, and is alsoan element effective for separating non-metallic inclusion, whichdeteriorates the stretch-flanging property, as a slug, so that it ispreferable to be included in an amount of not less than 0.01 mass %.When Al content is not more than 0.06 mass %, the above effect isobtained without the increase of cost for material. Therefore, Al ispreferable to be not more than 0.06 mass %, More preferably, it is arange of 0.02-0.06 mass %.

In addition to the above components, the cold-rolled steel sheet of theinvention may contain one or more selected from Nb: not more than 0.3mass %, Ti: not more than 0.3 mass %, V: not more than 0.3 mass %, Mo:not more than 0.3 mass %, Cr:

not more than 0.5 mass %, B: not more than 0.006 mass % and N: not morethan 0.008 mass %.

Nb, Ti and V are elements forming carbide and nitride to suppressferrite growth at a heating stage in the annealing and finely divide thestructure to improve the formability, particularly stretch-flangingproperty, and also Mo, Cr and B are elements improving the hardenabilityof steel and promoting the formation of bainite and martensite, so thatthey can be added within the above ranges. Also, N is an element formingnitrides with Nb, Ti and V or solid-soluting in steel to contribute tothe increase of the strength of steel, so that when it is not more than0.008 mass %, a greater amount of the nitride is not formed, and hencethe breakage due to the formation of voids in the press forming can besuppressed to obtain the above effect.

In addition to the above components, the cold-rolled steel sheet of theinvention may contain one or more selected from Ni: not more than 2.0mass %, Cu: not more than 2.0 mass %, Ca: not more than 0.1 mass % andREM: not more than 0.1 mass %.

Ni and Cu promote the formation of the low-temperature transformationphase to develop the effect of increasing the strength of steel, so thatthey can be added within the above ranges. Also, Ca and REM are elementscontrolling the form of the sulfide base inclusion to improve thestretch-flanging property of the steel sheet, so that they can be addedwithin the above ranges.

In the cold-rolled steel sheet of the invention, the remainder otherthan the above components is Fe and inevitable impurities. However,other components may be optionally added within a scope of not damagingthe action and effect of the invention.

The surface properties of the cold-rolled steel sheet of the inventionwill be described below.

As mentioned above, the cold-rolled steel sheet of the invention isnecessary to have a steel sheet surface obtained after the removal ofSi-containing oxide layer such as SiO₂ or Si—Mn based composite oxideformed on the surface layer of the steel sheet during annealing. Forthis end, it is necessary to conduct strong pickling with nitric acid orthe like to dissolve and remove the Si-containing oxide formed on thesurface of the steel sheet and in the grain boundary portion in thevicinity of the surface together with the steel matrix.

Furthermore, in the cold-rolled steel sheet of the invention, it isnecessary to reduce the ratio of covering the surface of the steel sheetwith iron-based oxide generated on the steel sheet surface by the strongpickling with nitric acid or the like to not more than 85% as an arearatio in addition to the removal of the Si-containing oxide layer. Whenthe surface covering ratio exceeds 85%, the dissolving reaction of ironin the phosphate treatment is inhibited to suppress the crystal growthof phosphate such as zinc phosphate or the like. However, in case ofusing a phosphate treating solution of a lower temperature, the coveringratio of not more than 85% is insufficient in cold-rolled steel sheetsused in applications requiring an extremely severe corrosion resistanceafter coating such as leg members for vehicle bodies particularlysubjected to severe corrosion, so that it should be further reduced tonot more than 40%, preferably not more than 35%.

In the invention, the surface covering ratio of the iron-based oxide isdetermined as follows:

The surface of the steel sheet after the pickling is observed at about 5fields with a ultra-low acceleration voltage scanning type electronmicroscope (ULV-SEM) capable of detecting information of an extremelysurface layer under conditions of acceleration voltage: 2 kV, operatingdistance: 3.0 mm and magnification: about 1000 times and spectroscopy isconducted with an energy dispersion type X-ray spectrometer (EDX) toobtain a reflection electron image. The reflection electron image isbinarized with an image analysis software, e.g. Image J to measure anarea ratio of a black portion. The measured results on the fields can beaveraged to obtain a surface covering ratio of the iron-based oxide.Moreover, as the ultra-low acceleration voltage scanning type electronmicroscope (ULV-SEM) may be mentioned, for example, ULTRA 55 made bySEISS, and as the energy dispersion type X-ray spectrometer (EDX) may bementioned, for example, NSS 312E made by Thermo Fisher.

Here, threshold value in the binarization will be described.

A steel slab of Steel symbol G shown in Table 3 of the following exampleis subjected to hot rolling, cold rolling and continuous annealing underconditions of No. 8 in Table 4 of the following example to obtain acold-rolled steel sheet of 1.8 mm in thickness, and then the cold-rolledsteel sheet after the continuous annealing is subjected to pickling andrepickling under conditions shown in Table 1, washed with water, driedand subjected to 0.7% temper rolling to obtain two cold-rolled steelsheets of Nos. a and b having different iron-based oxide amounts ontheir steel sheet surfaces. Then, the cold-rolled steel sheet of No. ais a standard sample having a large amount of iron-based oxide and thecold-rolled steel sheet of No. b is a standard sample having a smallamount of iron-based oxide, and each of these steel sheets is observedwith the scanning type electron microscope under the aforementionedconditions to obtain a reflection electron image. FIG. 1 showsphotographs of reflection electron images of steel sheets Nos. a and b,and FIG. 2 shows a histogram of pixel number to a gray value in thephotographs of the reflection electron images of the steel sheets Nos. aand b. In the invention, a gray value (Y point) corresponding to anintersecting point (X point) of the histograms of Nos. a and b shown inFIG. 2 is defined as a threshold value. Incidentally, when the surfacecovering ratio of the iron-based oxide in the steel sheets Nos. a and bis determined with the above threshold value, it is 85.3% in the steelsheet No. a and 25.8% in the steel sheet No. b.

TABLE 1 Surface Pickling conditions Repickling conditions covering AcidTreating Acid Treating ratio of Steel concentration Temperature timeconcentration Temperature time iron-based sheet (g/l) (° C.) (Seconds)(g/l) (° C.) (Seconds) oxide (%) a Nitric acid: 40 10 — — — 85.3 250 +Hydrochloric acid: 25 b Nitric acid: 40 10 Hydrochloric 40 30 25.8 150 +acid: 10 Hydrochloric acid: 15

In order to more improve the phosphate treatability and hence thecorrosion resistance in the cold-rolled steel sheet of the invention, itis preferable that the maximum thickness of the iron-based oxide is notmore than 150 nm in addition that the covering ratio of the iron-basedoxide produced on the steel sheet surface by repickling is not more than40%. When the maximum thickness of the iron-based oxide is not more than150 nm, the dissolving reaction of iron through the phosphate treatmentis not inhibited locally and also the precipitation of crystal ofphosphate such as zinc phosphate or the like is not inhibited locally.More preferably, it is not more than 130 nm.

The maximum thickness of the iron-based oxide is measured as follows.First, 10 extraction replicas are prepared from the surface of the steelsheet after the pickling by a focused ion beam (FIB) work for observinga section of about 8 pm relative to the widthwise direction of the steelsheet. Then, the section of 8 μm in the each replica is continuouslyshot by means of a transmission electron microscope (TEM) provided withan energy dispersion type X-ray spectrometer (EDX) capable of checkinglocal information of the section at an acceleration voltage of 200 kVand a magnification of 100000 times. As an example, FIG. 3 is aphotograph showing a section of a covering layer existing on the surfaceof the steel sheet and generated by pickling as observed by TEM, andFIG. 4 shows analytical results of the covering layer by EDX. As seenfrom FIG. 4, the covering layer is an iron-based oxide composed mainlyof iron. Therefore, the interval between a line A showing a surface ofthe steel sheet and a line B showing a thickest portion of an oxidelayer shown by the photograph of the section in FIG. 3 is measured withrespect to the 10 replicas, and a maximum thickness among them is amaximum thickness of the iron-based oxide. Moreover, the size andnumbers of the replicas, measuring conditions by TEM and the like asmentioned above are merely exemplified, and may be properly modified asa matter of course.

The production method of the cold-rolled steel sheet according to theinvention will be described below.

The production method of the cold-rolled steel sheet of the invention isnecessary to be a method wherein a steel material (slab) having Si:0.5-3.0 mass % is heated, hot rolled, cold rolled, continuously annealedand then strong-pickled with nitric acid or the like to removeSi-containing oxide layer on a surface layer portion of the steel sheetand further repickled to render a surface covering ratio of aniron-based oxide not more than 40% generated on the steel sheet surfaceby the above strong pickling. Further, it is preferable to be a methodwherein a maximum thickness of the iron-based oxide can be made to notmore than 150 nm. Therefore, the procedure ranging from the steel-makingstep to the continuous annealing step after the cold rolling can becarried out according to the usual manner, but the pickling after thecontinuous annealing is preferable to be conducted under the followingconditions.

Pickling Conditions After Continuous Annealing

On the surface layer of the steel sheet after the continuous annealingis produced a greater amount of the Si-containing oxide such as SiO₂,Si—Mn based composite oxide or the like, so that the phosphatetreatability and the corrosion resistance after coating are considerablydeteriorated. In the production method of the invention, therefore, itis necessary that the cold-rolled steel sheet after the annealing isstrongly pickled with nitric acid or the like, whereby the Si-containingoxide layer on the surface of the steel sheet is removed with the steelmatrix.

As previously mentioned, Si—Mn based composite oxide among theSi-containing oxides is easily dissolved in an acid, but SiO₂ isinsoluble in an acid. Therefore, in order to remove the Si-containingoxide including SiO₂, it is necessary to remove the oxide layer togetherwith the steel matrix of the steel sheet by the strong pickling. As theacid usable in the strong pickling can be preferably used nitric acid asa strong oxidizable acid, but hydrofluoric acid, hydrochloric acid,sulfuric acid or the like may be used as long as the Si-containing oxidelayer can be removed, so that the kind of the acid is particularly nomatter. Also, it is effective to add a pickling promoting agent to theacid, or to co-use an electrolytic treatment to promote the dissolutionof the steel matrix.

Moreover, in order to remove the Si-containing oxide layer from thesurface layer of the steel sheet after the continuous annealing andmitigate the load of the following repickling, it is preferable tosuppress the amount of the iron-based oxide generated on the steel sheetsurface by the strong pickling after the continuous annealing and beforethe repickling. For this end, it is preferable to conduct the picklingwith a pickling solution having a nitric acid concentration of more than50 g/L but not more than 200 g/L wherein hydrochloric acid having aneffect of breaking the oxide is mixed so that a ratio R (HCl/HNO₃) ofhydrochloric acid concentration to nitric acid concentration is a rangeof 0.01-1.0 or hydrofluoric acid is mixed so that a ratio (HF/HNO₃) ofhydrofluoric acid concentration to nitric acid concentration is a rangeof 0.01-1.0. In case of using the above pickling solution, it ispreferable that a temperature of the pickling solution is 20-70° C. anda pickling time is 3-30 seconds.

Repickling Conditions After the Pickling

However, when only the strong pickling is carried out with the picklingsolution obtained by mixing nitric acid and hydrofluoric acid or nitricacid and hydrofluoric acid as mentioned above, it is difficult to stablycontrol the surface covering ratio of the iron-based oxide generated onthe surface of the steel sheet to not more than 40%. In the invention,therefore, in order to more surely reduce the iron-based oxide generatedon the surface of the steel sheet by the strong pickling, the iron-basedoxide is dissolved and removed by further repickling the steel sheetpickled after the continuous annealing with a non-oxidizable acid.

The non-oxidizable acid usable in the repickling includes hydrochloricacid, sulfuric acid, phosphoric acid, pyrophosphoric acid, formic acid,acetic acid, citric acid, hydrofluoric acid, oxalic acid and a mixedacid of two or more thereof. Any of these may be used, but hydrochloricacid or sulfuring acid commonly used in the iron-making industry may bepreferably used. Among them, hydrochloric acid is preferable because itis a volatile acid and hardly remains a residue on the steel sheetsurface after washing with water different from sulfuric acid retainingsulfuric acid root and is large in the effect of breaking the oxide bychloride ion. Also, a mixed acid of hydrochloric acid and sulfuric acidmay be used.

When hydrochloric acid is used as the pickling solution in therepickling, it is preferable that a concentration of hydrochloric acidis 0.1-50 g/L, while in case of using sulfuric acid, it is preferablethat a concentration of sulfuric acid is 0.1-150 g/L. Also, when themixed acid of hydrochloric acid and sulfuric acid is used in therepickling, it is preferable to use a mixed acid having a hydrochloricacid concentration of 0.1-20 g/L and a sulfuric acid concentration of0.1-60 g/L. Also, the repickling of the invention is preferable to beconducted at a temperature of a repickling solution of 20-70° C. for atreating time of 1-30 seconds even in case of using any of therepickling solutions. When the concentration of the repickling solutionis more than the above lower limit and the liquid temperature is notlower than 20° C. and the treating time is not less than 1 second, it issufficient to remove the iron-based oxide existing on the steel sheetsurface, while when the concentration of the repickling solution is notmore than the above upper limit and the temperature is not higher than70° C. and the treating time is not more than 30 seconds, thedissolution of the steel sheet surface becomes not excessive and a newsurface oxide film is not formed.

In order to obtain steel sheets being more excellent in the phosphatetreatability and corrosion resistance, it is preferable that the maximumthickness of the iron-based oxide existing on the steel sheet surfaceafter the pickling is surely thinned to not more than 150 nm.

For this end, it is preferable to properly increase the concentration ofthe pickling solution used in the repickling. For example, it ispreferable that when hydrochloric acid is used in the repickling, theconcentration of hydrochloric acid is 3-50 g/L, while when sulfuric acidis used in the repickling, the concentration of sulfuric acid is 8-150g/L. On the other hand, when a mixture of hydrochloric acid and sulfuricacid is used as a pickling solution in the repickling, it is preferableto use a mixed acid having a hydrochloric acid concentration of 3-20 g/Land a sulfuric acid concentration of 8-60 g/L. In any case, when theacid concentration is within the above range, the iron-based oxide canbe surely thinned to not more than 150 nm, whereby the phosphatetreatability and the corrosion resistance after coating are improved.Also, when the acid concentration is within the above range, thedissolution of the steel sheet surface becomes not excessive, and hencenew surface oxide film is never formed.

The cold-rolled steel sheet, wherein the covering ratio of the steelsheet surface with the iron-based oxide is made to not more than 40% bypickling and repickling after the continuous annealing as mentionedabove, or alternately the cold-rolled steel sheet, wherein the maximumthickness of the iron-based oxide is made to not more than 150 nm, issubsequently subjected to usual treating steps such as temper rollingand the like to provide products.

EXAMPLE 1

A steel comprising C: 0.125 mass %, Si: 1.5 mass %, Mn: 2.6 mass %, P:0.019 mass %, S: 0.008 mass %, Al: 0.040 mass % and the remainder beingFe and inevitable impurities is prepared according to common refiningprocess such as melting in a converter, degassing treatment and the likeand continuously cast into a steel material (slab). Then, the slab isreheated to a temperature of 1150-1170° C., hot rolled at a terminatingtemperature of finish rolling of 850-880° C. and coiled at a temperatureof 500-550° C. to obtain a hot-rolled steel sheet having a thickness of3-5 mm. Then, the hot-rolled steel sheet is pickled to remove scales andthereafter cold rolled to obtain a cold-rolled steel sheet having athickness of 1.8 mm. Next, the cold-rolled steel sheet is subjected tosuch a continuous annealing that it is heated to a soaking temperatureof 750-780° C. and held at this temperature for 40-50 seconds and thencooled at a rate of 20-30° C./second from the soaking temperature to acooling stop temperature of 350-400° C. and held at the cooling stoptemperature range for 100-120 seconds, and then the steel sheet ispickled and further repickled under conditions shown in Table 2, washedwith water, dried and subjected to a temper rolling at a stretchingratio of 0.7% to obtain cold-rolled steel sheets Nos. 1-85 shown inTable 2.

A test specimen is sampled from each of the above cold-rolled steelsheets and observed at 5 fields of the steel sheet surface with ascanning type electron micrcope of ultra-low acceleration voltage(ULV-SEM; made by SEISS; ULTRA 55) at an acceleration voltage of 2 kV,an operating distance of 3.0 mm and a magnification of 1000 times. Andanalyzed with an energy dispersion X-ray spectrometer (EDX; made byThermo Fisher; NSS 312E) to obtain a reflection electron image. Thereflection electron image is binarized with an image analyzing software(Image J) with respect to gray value (Y-point) corresponding tointersect point (X-point) and threshold value defined in histograms ofthe aforementioned standard samples Nos. a and b to measure an arearatio of a black portion. The values measured at 5 fields are averagedas a surface covering ratio of iron-based oxide.

Also, a test specimen is sampled from each of the above cold-rolledsteel sheets and subjected to a phosphate treatment and a coatingtreatment under the following conditions and then subjected to threecorrosion tests of hot salt water immersion test, salt water spray testand composite cycle corrosion test to evaluate a corrosion resistanceafter coating. Further, a distribution of O, Si, Mn and Fe in depthdirection on the surface of the test specimen sampled from eachcold-rolled steel sheet is measured with GDS.

(1) Phosphate Treating Conditions

The test specimen sampled from each cold-rolled steel sheet is subjectedto a phosphate treatment with a degreasing agent: FC-E2011, a surfaceregulator: PL-X and a phosphate treating agent: PALBOND PB-L3065, whichare made by Nihon Parkerizing Co., Ltd., so as to provide a phosphatecoating adhered amount of 1.7-3.0 g/m² under two conditions of thefollowing standard condition and comparative condition of lowering thephosphate treating temperature to a low temperature.

<Standard Condition>

-   -   Degreasing step: treating temperature 40° C., treating time 120        seconds    -   Spray degreasing, surface regulating step: pH 9.5, Treating        temperature room temperature, treating time 20 seconds    -   Phosphate treating step: temperature of phosphate treating        solution 35° C., treating time 120 seconds

<Low Temperature Condition>

Condition of lowering the temperature of the phosphate treating solutionin the above standard condition to 33° C.

(2) Corrosion Test

The surface of the test specimen subjected to the phosphate treatment iselectrodeposited with an electrodeposition paint : V-50 made by NipponPaint Co., Ltd. so as to have a coating thickness of 25 μm and thensubjected to the following three corrosion tests.

<Hot Salt Water Immersion Test>

The test specimen (n=1) subjected to the phosphate treatment andelectrodeposition is provided on its surface with a crosscut flaw of 45mm in length by means of a cutter, and thereafter immersed in a solutionof 5 mass % NaCl (60° C.) for 360 hours, washed with water, and dried.After an adhesive tape is attached to a cut flaw portion, a test ofpeeling off the tape is carried out to measure a maximum peeled fullwidth combining either side of the cut flaw portion. When the maximumpeeled full width is not more than 5.0 mm, the corrosion resistance canbe evaluated to be good in the hot slat water immersion test.

<Salt Water Spray Test (SST)>

The test specimen (n=1) subjected to the phosphate treatment andelectrodeposition is provided on its surface with a crosscut flaw of 45mm in length by means of a cutter, and thereafter subjected to a saltwater spray test with an aqueous solution of 5 mass % NaCl for 1200hours according to a neutral salt water spray test defined in JISZ2371:2000, and then a tape peeling test on a crosscut flaw portion isconducted to measure a maximum peeled full width combining either sideof the cut flaw portion. When the maximum peeled full width is not morethan 4.0 mm, the corrosion resistance can be evaluated to be good in thesalt water spray test.

<Composite Cycle Corrosion Test (CCT)>

The test specimen (n=1) subjected to the phosphate treatment andelectrodeposition is provided on its surface with a crosscut flaw of 45mm in length by means of a cutter, and thereafter subjected to acorrosion test that one cycle of salt water spraying (aqueous solutionof 5 mass % NaCl: 35° C., relative humidity: 98%) for 2 hours →drying(60° C., relative humidity: 30%) for 2 hours wetting (50° C., relativehumidity: 95%) for 2 hours is repeated 120 cycles, washed with water anddried, and then a tape peeling test on a cut flaw portion is conductedto measure a maximum peeled full width combining either side of the cutflaw portion. When the maximum peeled full width is not more than 6.0mm, the corrosion resistance can be evaluated to be good in thecomposite cycle corrosion test.

The test results are also shown in Table 2. As seen from these results,the steel sheets of Invention Examples subjected to the pickling andrepickling under the conditions adequate for the invention after thecontinuous annealing are small in the maximum peeled full width on allof the hot salt water immersion test, salt water spray test andcomposite cycle corrosion test and show the good corrosion resistanceafter coating. Particularly, all of the cold-rolled steel sheets havingthe surface covering ratio of the iron-based oxide of not more than 40%are excellent in the corrosion resistance after coating under severecorrosion environment. Moreover, as the distribution in depth directionof O, Si, Mn and Fe on the surface of each steel sheet in Table 2 ismeasured with GDS, it has been confirmed that in the steel sheetspickled under the conditions adequate for the invention, peaks of Si andO do not appear and the Si-containing oxide layer is removedsufficiently. As a reference, FIG. 5 shows the profile in depthdirection of O, Si, Mn and Fe as surface-analyzed with GDS with respectto the test specimens of Comparative Example No. 1 and Invention ExampleNo. 9 in Table 2.

TABLE 2-1 Surface properties Surface Pickling condition Repicklingcondition covering Acid Treating Acid Treating ratio of concentrationTemperature time concentration Temperature time iron-based No (g/l) (°C.) (seconds) (g/l) (° C.) (seconds) oxide (%) 1 Nitric acid: 40 10 — —— 72.6 2 150 + Hydrochloric 40 1 39.5 3 Hydrochloric acid: 0.1 10 35.3 4acid: 15 30 30.4 5 Hydrochloric 20 1 39.1 6 acid: 10 10 36.1 7 30 32.3 8Hydrochlori 40 1 35.2 9 acid: 10 10 30.3 10 30 25.9 11 Hydrochloric 70 130.9 12 acid: 10 10 25.1 13 30 22.3 14 Hydrochloric 40 1 30.1 15 acid:50 10 26.2 16 30 21.2 17 Hydrochloric 40 1 49.8 18 acid: 100 10 54.5 1930 59.8 20 Nitric acid: 40 10 Hydrochloric 40 1 39.7 21 50 + acid: 0.110 36.1 22 Hydrofluoric 30 32.1 23 acid: 50 Hydrochloric 20 1 39.5 24acid: 10 10 37.2 25 30 33.6 26 Hydrochloric 40 1 36.2 27 acid: 10 1032.4 28 30 28.3 29 Hydrochloric 70 1 32.1 30 acid: 10 10 26.8 31 30 24.132 Hydrochloric 40 1 31.2 33 acid: 50 10 25.6 34 30 22.3 35 Hydrochloric40 1 45.9 36 acid: 100 10 55.3 37 30 62.1 Full width peeled aftercorrosion test (mm) Temperature of phosphate treating solution 35° C.33° C. Hot salt water Salt water Composite cycle No Immersion test Spraytest corrosion test Remarks 1 6.3 5.5 7.9 8.3 Comparative example 2 4.94.0 5.8 5.8 Invention example 3 4.6 3.8 5.6 5.6 Invention example 4 4.43.6 5.0 5.0 Invention example 5 4.9 4.0 5.8 5.8 Invention example 6 4.83.7 5.4 5.5 Invention example 7 4.6 3.6 5.2 5.0 Invention example 8 4.63.7 5.3 5.6 Invention example 9 4.5 3.6 4.8 5.0 Invention example 10 4.03.1 4.4 4.5 Invention example 11 4.4 3.5 4.8 4.9 Invention example 124.0 3.2 4.1 4.5 Invention example 13 3.7 3.0 4.0 4.1 Invention example14 4.3 3.5 4.8 4.8 Invention example 15 4.1 3.2 4.3 4.5 Inventionexample 16 3.5 3.0 3.6 3.6 Invention example 17 5.5 4.4 6.7 6.8Comparative example 18 5.7 4.7 7.1 7.4 Comparative example 19 5.9 5.17.3 7.6 Comparative example 20 4.9 3.9 5.9 5.8 Invention example 21 4.83.8 5.4 5.4 Invention example 22 4.2 3.5 5.0 5.0 Invention example 235.0 4.0 5.6 5.6 Invention example 24 4.7 3.9 5.6 5.4 Invention example25 4.6 3.6 5.3 5.1 Invention example 26 4.8 3.8 5.5 5.8 Inventionexample 27 4.6 3.7 5.3 5.5 Invention example 28 4.2 3.5 4.8 4.6Invention example 29 4.6 3.6 5.0 5.3 Invention example 30 4.2 3.3 4.54.6 Invention example 31 3.9 3.2 4.2 4.3 Invention example 32 4.3 3.54.6 5.0 Invention example 33 4.0 3.3 4.1 4.6 Invention example 34 3.73.1 3.6 4.2 Invention example 35 5.3 4.2 6.2 6.4 Comparative example 365.8 4.7 7.1 7.3 Comparative example 37 6.0 5.1 7.3 7.7 Comparativeexample

TABLE 2-2 Surface properties Surface Pickling condition Repicklingcondition covering Acid Treating Acid Treating ratio of concentrationTemperature time concentration Temperature time iron-based No (g/l) (°C.) (seconds) (g/l) (° C.) (seconds) oxide (%) 38 Nitric acid: 40 10Sulfuric 40 1 39.5 39 150 + acid: 0.1 10 35.3 40 Hydrochloric 30 30.4 41acid: 15 Sulfuric 20 1 39.1 42 acid: 75 10 36.1 43 30 32.3 44 Sulfuric40 1 35.2 45 acid: 75 10 30.3 46 30 25.9 47 Sulfuric 70 1 30.9 48 acid:75 10 25.1 49 30 22.3 50 Sulfuric 40 1 30.1 51 acid: 150 10 26.2 52 3021.2 53 Sulfuric 40 1 49.9 54 acid: 200 10 55.0 55 30 62.1 56 Nitricacid: 40 10 Sulfuric 40 1 39.7 57 50 + acid: 0.1 10 36.1 58 Hydrofluoric30 32.1 59 acid: 50 Sulfuric 20 1 39.5 60 acid: 75 10 37.2 61 30 33.6 62Sulfuric 40 1 36.2 63 acid: 75 10 32.4 64 30 28.3 65 Sulfuric 70 1 32.166 acid: 75 10 26.8 67 30 24.1 68 Sulfuric 40 1 31.2 69 acid: 150 1025.6 70 30 22.3 71 Sulfuric 40 1 50.1 72 acid: 200 10 55.3 73 30 61.5Full width peeled after corrosion test (mm) Temperature of phosphatetreating solution 35° C. 33° C. Hot salt water Salt water Compositecycle No Immersion test Spray test corrosion test Remarks 38 4.8 4.0 5.75.9 Invention example 39 4.7 3.9 5.6 5.7 Invention example 40 4.6 3.75.1 5.2 Invention example 41 4.8 4.1 5.9 5.9 Invention example 42 4.73.8 5.6 5.6 Invention example 43 4.5 3.7 5.2 5.4 Invention example 444.8 3.9 5.6 5.6 Invention example 45 4.6 3.6 5.2 5.1 Invention example46 4.3 3.3 4.8 4.8 Invention example 47 4.6 3.8 5.2 5.2 Inventionexample 48 4.2 3.5 4.7 4.7 Invention example 49 3.9 3.2 4.5 4.6Invention example 50 4.6 3.5 5.2 5.2 Invention example 51 4.3 3.3 4.64.8 Invention example 52 4.0 3.2 4.2 4.5 Invention example 53 5.4 4.46.6 6.8 Comparative example 54 5.7 4.7 7.1 7.4 Comparative example 556.0 5.2 7.4 7.6 Comparative example 56 5.0 3.8 5.9 6.0 Invention example57 4.7 3.7 5.7 5.8 Invention example 58 4.7 3.6 5.5 5.6 Inventionexample 59 4.9 4.2 6.0 6.0 Invention example 60 4.8 4.0 5.8 5.8Invention example 61 4.5 3.7 5.6 5.6 Invention example 62 4.8 3.9 5.65.7 Invention example 63 4.6 3.6 5.3 5.4 Invention example 64 4.5 3.54.9 5.2 Invention example 65 4.5 3.7 5.3 5.3 Invention example 66 4.33.4 4.6 4.9 Invention example 67 4.2 3.3 4.4 4.7 Invention example 684.5 3.6 5.2 5.3 Invention example 69 4.2 3.5 4.8 4.7 Invention example70 4.2 3.3 4.4 4.6 Invention example 71 5.4 4.5 6.6 6.8 Comparativeexample 72 5.8 4.8 7.2 7.5 Comparative example 73 5.9 5.2 7.4 7.7Comparative example

TABLE 2-3 Surface properties Surface Pickling condition Repicklingcondition covering Acid Treating Acid Treating ratio of concentrationTemperature time concentration Temperature time iron-based No (g/l) (°C.) (seconds) (g/l) (° C.) (seconds) oxide (%) 74 Nitric acid: 40 10Hydrochloric 40 1 35.5 75 150 + acid: 5 + 10 30.6 76 HydrochloricSulfuric 30 26.3 acid: 15 acid: 5 77 Nitric acid: 40 10 Hydrochloric 401 33.2 78 150 + acid: 10 + 10 30.1 79 Hydrochloric Sulfuric 30 25.6acid: 15 acid: 50 80 Nitric acid: 40 10 Hydrochloric 40 1 35.7 81 50 +acid: 5 + 10 30.9 82 Hydrofluoric Sulfuric 30 27.0 acid: 50 acid: 5 83Nitric acid: 40 10 Hydrochloric 40 1 34.6 84 50 + acid: 5 + 10 30.1 85Hydrofluoric Sulfuric 30 26.5 acid: 50 acid: 5 Full width peeled aftercorrosion test (mm) Temperature of phosphate treating solution 35° C.33° C. Hot salt water Salt water Composite cycle No Immersion test Spraytest corrosion test Remarks 74 4.5 3.7 5.4 5.8 Invention example 75 4.43.5 4.9 5.0 Invention example 76 3.9 3.2 4.6 4.6 Invention example 774.4 3.6 5.3 5.8 Invention example 78 4.2 3.5 4.9 5.0 Invention example79 3.8 3.3 4.5 4.5 Invention example 80 4.6 3.8 5.4 5.7 Inventionexample 81 4.5 3.6 4.9 5.3 Invention example 82 4.1 3.2 4.7 4.8Invention example 83 4.5 3.8 5.2 5.6 Invention example 84 4.4 3.5 5.05.2 Invention example 85 4.1 3.1 4.6 4.7 Invention example

EXAMPLE 2

Each of steels A-X having a chemical composition shown in Table 3 isprepared according to common refining process such as melting in aconverter, degassing treatment and the like and continuously cast into asteel slab. The steel slab is hot rolled under hot rolling conditionsshown in Table 4 to obtain a hot-rolled steel sheet having a thicknessof 3-4 mm, which is pickled to remove scales on the surface of the steelsheet and thereafter cold rolled to obtain a cold-rolled steel sheethaving a thickness of 1.8 mm. Next, the cold-rolled steel sheet iscontinuously annealed under the conditions shown in Table 4, pickled andrepickled under conditions shown in Table 5, washed with water, driedand subjected to a temper rolling at a stretching ratio of 0.7% toobtain cold-rolled steel sheets Nos. 1-39.

TABLE 3 Chemical composition (mass %) Steel Nb, Ti, V, Mo, Ni, Cu,Symbol C Si Mn P S Al Si/Mn Cr, B, N Ca, REM A 0.11 1.25 1.55 0.0180.001 0.032 0.81 — — B 0.15 1.30 1.80 0.019 0.002 0.033 0.72 — — C 0.151.20 1.95 0.017 0.001 0.033 0.62 — — D 0.09 1.45 1.40 0.017 0.002 0.0281.04 — — E 0.18 1.11 1.36 0.018 0.001 0.032 0.82 — — F 0.16 1.41 1.230.017 0.001 0.041 1.15 — — G 0.14 1.65 1.33 0.018 0.002 0.035 1.24 — — H0.12 1.45 2.10 0.017 0.001 0.042 0.69 — — I 0.17 0.90 1.40 0.017 0.0020.044 0.64 — — J 0.13 1.20 1.89 0.018 0.001 0.041 0.63 — — K 0.15 1.201.85 0.017 0.001 0.034 0.65 — — L 0.03 1.25 3.25 0.018 0.001 0.005 0.38— — M 0.22 3.30 1.15 0.018 0.001 0.027 2.87 — — N 0.06 1.28 2.12 0.0250.003 0.040 0.60 Nb: 0.1, Cu: 0.15 Ti: 0.2 O 0.18 1.21 1.97 0.015 0.0020.035 0.61 V: 0.1, Ni: 0.13 Mo: 0.2 P 0.18 1.56 2.58 0.010 0.002 0.0300.60 Cr: 0.2, Ca: 0.003 B: 0.005 Q 0.13 1.32 1.32 0.030 0.001 0.040 1.00N: 0.007 REM: 0.002 R 0.07 1.26 2.10 0.025 0.002 0.040 0.60 Nb: 0.1 — S0.06 1.28 2.12 0.025 0.003 0.040 0.60 Nb: 0.1, — Ti: 0.2 T 0.17 1.231.99 0.015 0.002 0.050 0.62 — Ni: 0.13 U 0.18 1.22 1.97 0.015 0.0030.040 0.62 — Ni: 0.13, Ca: 0.003 V 0.18 1.21 1.98 0.015 0.002 0.035 0.61V: 0.1 Ni: 0.13 W 0.18 1.56 2.58 0.010 0.002 0.030 0.60 Mo: 0.1, Ca:0.003 Cr: 0.2, B: 0.005 X 0.13 1.32 1.32 0.030 0.001 0.040 1.00 Nb: 0.1,Cu: 0.2, N: 0.007 REM: 0.002 Y 0.01 0.02 0.25 0.020 0.012 0.040 0.08 — —Z 0.11 0.45 1.50 0.020 0.003 0.030 0.30 — —

TABLE 4-1 Hot rolling conditions Cold Continuous annealing conditionsHeating Finish Cooling Coiling rolling Heating Holding Cooling SteelTemperature Temperature rate temperature reduction temperature time rateNo symbol (° C.) (° C.) (° C./s) (° C.) (%) (° C.) (Second) (° C./s) 1 A1150 850 25 620 60 780 45 20 2 B 1150 820 31 400 60 780 40 20 3 B 1150820 31 400 60 780 40 20 4 C 1140 850 26 600 60 760 50 20 5 D 1150 840 33530 60 730 40 20 6 E 1150 850 30 580 55 750 35 20 7 F 1150 850 25 620 60750 50 20 8 G 1150 850 33 550 60 750 30 20 9 G 1150 850 33 550 60 750 3020 10 G 1150 850 33 550 60 750 30 20 11 G 1150 850 33 550 60 750 30 2012 G 1150 850 33 550 60 750 30 20 13 H 1130 820 28 570 60 780 50 15 14 I1150 840 34 530 55 780 50 15 15 J 1140 850 28 600 60 770 60 20 16 K 1150850 25 620 60 780 45 20 17 L 1100 850 33 550 60 750 50 20 18 L 1100 85033 550 60 750 50 20 19 L 1100 850 33 550 60 750 50 20 20 L 1100 850 33550 60 750 50 20 21 L 1100 850 33 550 60 750 50 20 22 M 1120 830 31 55055 720 50 15 Continuous annealing conditions Cooling stop HoldingCooling Strength temperature time rate TS No (° C.) (second) (° C./s)(MPa) Remarks 1 350 100 40 625 Invention example 2 400 100 50 821Invention example 3 400 100 50 819 Invention example 4 350 100 45 814Invention example 5 350 110 40 623 Invention example 6 400 110 50 836Invention example 7 350 120 50 634 Invention example 8 400 100 50 632Comparative example 9 400 100 50 635 Invention example 10 400 100 50 631Invention example 11 400 100 50 633 Invention example 12 400 100 50 634Comparative example 13 370 150 50 840 Invention example 14 350 120 55812 Invention example 15 300 100 45 836 Invention example 16 350 100 40650 Invention example 17 450 150 50 960 Comparative example 18 450 15050 959 Invention example 19 450 150 50 963 Invention example 20 450 15050 962 Invention example 21 450 150 50 961 Comparative example 22 410190 50 1124 Comparative example

TABLE 4-2 Hot rolling conditions Cold Continuous annealing conditionsHeating Finish Cooling Coiling rolling Heating Holding Cooling SteelTemperature Temperature rate temperature reduction temperature time rateNo symbol (° C.) (° C.) (° C./s) (° C.) (%) (° C.) (Second) (° C./s) 23N 1120 830 33 550 60 750 30 20 24 O 1150 850 32 560 60 750 35 20 25 P1130 840 33 550 55 780 30 20 26 Q 1140 850 33 580 60 750 40 20 27 R 1120830 33 550 60 750 30 20 28 S 1120 830 32 550 60 750 35 20 29 T 1150 85032 560 60 750 35 20 30 U 1140 850 33 550 60 750 35 20 31 V 1150 850 32550 60 750 40 20 32 W 1130 840 33 550 55 780 30 20 33 X 1140 850 33 58060 750 40 20 34 Y 910 630 22 550 80 750 40 20 35 Y 905 650 24 560 83 75040 20 36 Y 910 640 24 550 85 750 35 20 37 Z 990 690 25 540 75 750 35 2038 Z 970 710 28 540 70 750 35 20 Continuous annealing conditions Coolingstop Holding Cooling Strength temperature time rate TS No (° C.)(second) (° C./s) (MPa) Remarks 23 400 100 50 613 Invention example 24350 100 50 776 Invention example 25 400 110 50 1152 Invention example 26400 120 45 586 Invention example 27 400 100 50 611 Invention example 28410 100 50 621 Invention example 29 350 100 50 773 Invention example 30400 100 50 785 Invention example 31 380 100 50 770 Invention example 32400 110 50 1156 Invention example 33 400 120 45 585 Invention example 34370 100 50 285 Invention example 35 400 100 50 279 Invention example 36400 100 50 290 Invention example 37 400 100 50 785 Invention example 38400 100 50 790 Invention example

TABLE 5-1 Surface properties Surface Pickling conditions Repicklingconditions covering Acid Treating Acid Treating ratio of Steelconcentration Temperature time concentration Temperature time iron-basedNo Symbol (g/l) (° C.) (seconds) (g/l) (° C.) (seconds) Oxide (%) 1 ANitric Acid: 40 10 Hydrochloric 40 10 30.1 150 + Acid: 1 2 BHydrochloric 40 10 Hydrochloric 40 10 30.5 Acid: 15 Acid: 10 3 BNitricAcid: 40 10 Hydrochloric 40 10 30.2 50 + Acid: 10 Hydrofluoricacid: 50 4 C Nitric acid: 40 10 Hydrochloric 40 10 29.9 150 + Acid: 10 5D Hydrochloric 40 10 Hydrochloric 40 10 30.7 acid: 15 Acid: 10 6 E 40 10Hydrochloric 40 10 30.2 Acid: 10 7 F 40 10 Hydrochloric 40 10 30.3 Acid:10 8 G 40 10 Hydrochloric 10 1 74.3 Acid: 10 9 G 40 10 Hydrochloric 40 135.5 Acid: 10 10 G 40 10 Hydrochloric 40 30 25.8 Acid: 10 11 G 40 10Sulfuric 40 30 26.4 Acid: 75 12 G 40 10 Hydrochloric 40 10 54.5 Acid:100 13 H 40 10 Hydrochloric 40 10 30.5 Acid: 10 14 I 40 10 Hydrochloric40 10 30.8 Acid: 10 15 J 40 10 Hydrochloric 40 10 29.8 Acid: 10 16 K 4010 Hydrochloric 40 10 30.1 Acid: 10 17 L 40 10 Sulfuric 10 1 75.2 Acid:75 18 L 40 10 Sulfuric 40 1 35.3 Acid: 75 19 L 40 10 Sulfuric 40 30 25.5Acid: 75 20 L 40 10 Hydrochloric 40 30 25.4 Acid: 10 21 L 40 10 Sulfuric40 10 55.0 Acid: 200 22 M 40 10 Hydrochloric 40 10 41.2 Acid: 10 Fullwidth peeled after corrosion test(mm) Temperature of phosphate treatingsolution: 35° C. 33° C. Hot salt water Salt water Composite cycle Noimmersion test spray test corrosion test Remarks 1 4.4 3.7 4.8 4.9Invention example 2 4.3 3.7 4.6 5.0 Invention example 3 4.5 3.8 5.1 5.3Invention example 4 4.4 3.6 4.7 5.2 Invention example 5 4.4 3.9 4.9 5.2Invention example 6 4.3 3.8 4.8 5.3 Invention example 7 4.6 3.5 4.8 5.1Invention example 8 6.5 5.3 7.7 8.0 Comparative example 9 4.5 3.9 5.25.5 Invention example 10 4.0 3.0 4.5 4.6 Invention example 11 4.3 3.34.8 4.9 Invention example 12 5.7 4.7 7.1 7.4 Comparative example 13 4.23.9 4.8 5.2 Invention example 14 4.2 3.8 5.0 5.2 Invention example 154.3 3.9 4.9 5.1 Invention example 16 4.1 4.0 4.7 5.2 Invention example17 6.4 5.5 7.8 8.2 Comparative example 18 4.4 3.9 5.3 5.4 Inventionexample 19 4.4 3.3 4.9 5.2 Invention example 20 4.5 3.2 5.0 5.1Invention example 21 5.7 4.7 7.1 7.4 Comparative example 22 5.2 4.1 6.36.5 Comparative example

TABLE 5-2 Surface properties Surface Pickling conditions Repicklingconditions covering Acid Treating Acid Treating ratio of Steelconcentration Temperature time concentration Temperature time iron-basedNo Symbol (g/l) (° C.) (seconds) (g/l) (° C.) (seconds) Oxide (%) 23 NNitricacid: 40 10 Hydrochloric 40 10 30.8 150 + acid: 10 24 OHydrochloric 40 10 Hydrochloric 40 10 31.3 acid: 15 acid: 10 25 P 40 10Hydrochloric 40 10 30.9 acid: 10 26 Q Nitric acid: 40 10 Hydrochloric 4010 31.0 50 + acid: 10 27 R Hydrochloric 40 10 Hydrochloric 40 10 30.7acid: 5 acid: 10 28 S Nitric acid: 40 10 Hydrochloric 40 10 31.1 150 +acid: 10 29 T Hydrochloric 40 10 Hydrochloric 40 10 31.4 acid: 15 acid:10 30 U Nitric acid: 40 10 Hydrochloric 40 10 31.4 50 + acid: 10 31 VHydrochloric 40 10 Hydrochloric 40 10 30.9 acid: 5 acid: 10 32 W Nitricacid: 40 10 Hydrochloric 40 10 30.5 150 + acid: 10 33 X Hydrochloric 4010 Hydrochloric 40 10 31.4 acid: 15 acid: 10 34 Y 40 10 Hydrochloric 4010 29.8 acid: 10 35 Y 40 10 Hydrochloric 40 10 29.3 acid: 10 36 Y 40 10Hydrochloric 40 10 28.3 acid: 10 37 Z 40 10 Hydrochloric 40 10 37.4acid: 10 38 Z 40 10 Hydrochloric 40 10 35.3 acid: 10 39 Z 40 10Hydrochloric 40 10 29.9 acid: 10 Full width peeled after corrosiontest(mm) Temperature of phosphate treating solution: 35° C. 33° C. Hotsalt water Salt water Composite cycle No immersion test spray testcorrosion test Remarks 23 4.4 3.9 5.1 5.2 Invention example 24 4.3 3.95.1 5.3 Invention example 25 4.4 3.7 5.0 5.2 Invention example 26 4.43.8 4.9 5.1 Invention example 27 4.4 3.8 5.2 5.3 Invention example 284.4 4.0 5.1 5.2 Invention example 29 4.3 3.8 5.2 5.3 Invention example30 4.3 3.8 5.3 5.3 Invention example 31 4.4 3.9 5.2 5.3 Inventionexample 32 4.2 3.7 5.1 5.1 Invention example 33 4.3 3.9 4.9 5.2Invention example 34 4.2 3.6 4.7 5.2 Invention example 35 4.3 3.5 4.75.2 Invention example 36 4.1 3.5 4.5 5.1 Invention example 37 4.4 3.95.3 5.4 Invention example 38 4.3 3.9 5.3 5.3 Invention example 39 4.43.7 4.7 5.2 Invention example

A test specimen is sampled from each of the cold-rolled steel sheets andsubjected to the following tensile test and test for the corrosionresistance after coating after the surface covering ratio of iron-basedoxide on the steel sheet surface after the repickling is measured in thesame manner as in Example 1. Also, the distribution in depth directionof O, Si, Mn and Fe on the surface of the test specimen sampled fromeach of the cold-rolled steel sheets is measured with GDS.

(1) Mechanical Properties

A tensile test specimen of JIS No. 5 (n=1) sampled in a direction(C-direction) parallel to the rolling direction according to JISZ2201:1998 is subjected to a tensile test according to JIS Z2241:1998 tomeasure tensile strength TS.

(2) Corrosion Resistance After Coating

A test specimen is prepared by subjecting the test specimen sampled fromeach of the cold-rolled steel sheet to phosphate treatment andelectrodeposition under the same conditions as in Example 1 and thensubjected to three corrosion tests of hot salt water immersion test,salt water spray test (SST) and composite cycle corrosion test (CCT)likewise Example 1 to evaluate the corrosion resistance after coating.

The results of the above tests are shown in Tables 4 and 5. As seen fromthese results, the high-strength cold-rolled steel sheets of InventionExamples containing Si of not less than 0.5 mass % and pickled andrepickled under the conditions adequate for the invention to render thesurface covering ratio of the iron-based oxide into not more than 40%are excellent in the corrosion resistance after coating but also have atensile strength TS of not less than 590 MPa. Moreover, as thedistribution in depth direction of O, Si, Mn and Fe is measured withGDS, it has been confirmed that in all of the steel sheets pickled underthe conditions adequate for the invention, peaks of Si and O do notappear and the Si-containing oxide layer is removed sufficiently.

EXAMPLE 3

A steel comprising C: 0.125 mass %, Si: 1.5 mass %, Mn: 2.6 mass %, P:0.019 mass %, S: 0.008 mass %, Al: 0.040 mass % and the remainder beingFe and inevitable impurities is melted and continuously cast into asteel material (slab). The slab is reheated to a temperature of1150-1170° C., hot rolled at a terminating temperature of finish rollingof 850-880° C. and coiled at a temperature of 500-550° C. to obtain ahot-rolled steel sheet having a thickness of 3-4 mm. The hot-rolledsteel sheet is pickled to remove scales and thereafter cold rolled toobtain a cold-rolled steel sheet having a thickness of 1.8 mm. Next, thecold-rolled steel sheet is subjected to such a continuous annealing thatit is heated to a soaking temperature of 750-780° C. and held at thistemperature for 40-50 seconds and then cooled at a rate of 20-30°C./second from the soaking temperature to a cooling stop temperature of350-400° C. and held at the cooling stop temperature range for 100-120seconds, and then the steel sheet is pickled and repickled underconditions shown in Table 6, washed with water, dried and subjected to atemper rolling at a stretching ratio of 0.7% to obtain cold-rolled steelsheets Nos. 1-61 shown in Table 6.

A test specimen is sampled from each of the above cold-rolled steelsheets to measure a surface covering ratio and maximum thickness ofiron-based oxide generated on the surface of the steel sheet by picklingthrough the aforementioned methods.

Also, the test specimen is sampled from each of the above cold-rolledsteel sheets and subjected to phosphate treatment and coating treatmentunder the following conditions and then subjected to three corrosiontests of hot salt water immersion test, salt water spray test andcomposite cycle corrosion test to evaluate the corrosion resistanceafter coating. Further, the distribution in depth direction of O, Si, Mnand Fe on the surface of the test specimen sampled from each of thecold-rolled steel sheets is measured with GDS.

(1) Phosphate Treating Conditions

The test specimen sampled from each cold-rolled steel sheet is subjectedto a phosphate treatment with a degreasing agent: FC-E2011, a surfaceregulator: PL-X and a phosphate treating agent: PALBOND PB-L3065, whichare made by Nihon Parkerizing Co., Ltd., so as to provide a phosphatecoating adhered amount of 1.7-3.0 g/m2 under two conditions of thefollowing standard condition and comparative condition of lowering thephosphate treating temperature to a low temperature.

<Standard Condition>

-   -   Degreasing step: treating temperature 40° C., treating time 120        seconds    -   Spray degreasing, surface regulating step: pH 9.5, Treating        temperature room temperature, treating time 20 seconds    -   Phosphate treating step: temperature of phosphate treating        solution 35° C., treating time 120 seconds

<Low Temperature Condition>

Condition of lowering the temperature of the phosphate treating solutionin the above standard condition to 33° C.

(2) Corrosion Test

The surface of the test specimen subjected to the phosphate treatment iselectrodeposited with an electrodeposition paint : V-50 made by NipponPaint Co., Ltd. so as to have a coating thickness of 25 μm and thensubjected to the following three corrosion tests under more strictconditions than the one with Example 1.

<Hot Salt Water Immersion Test>

The test specimen (n=1) subjected to the phosphate treatment andelectrodeposition is provided on its surface with a crosscut flaw of 45mm in length by means of a cutter, and thereafter immersed in a solutionof 5 mass % NaCl (60° C.) for 480 hours, washed with water, and dried.After an adhesive tape is attached to a cut flaw portion, a test ofpeeling off the tape is carried out to measure a maximum peeled fullwidth combining either side of the cut flaw portion. When the maximumpeeled full width is not more than 5.0 mm, the corrosion resistance canbe evaluated to be good in the hot slat water immersion test.

<Salt Water Spray Test (SST)>

The test specimen (n=1) subjected to the phosphate treatment andelectrodeposition is provided on its surface with a crosscut flaw of 45mm in length by means of a cutter, and thereafter subjected to a saltwater spray test with an aqueous solution of 5 mass % NaCl for 1400hours according to a neutral salt water spray test defined in JISZ2371:2000, and then a tape peeling test on a crosscut flaw portion isconducted to measure a maximum peeled full width combining either sideof the cut flaw portion. When the maximum peeled full width is not morethan 4.0 mm, the corrosion resistance can be evaluated to be good in thesalt water spray test.

<Composite Cycle Corrosion Test (CCT)>

The test specimen (n=1) subjected to the phosphate treatment andelectrodeposition is provided on its surface with a crosscut flaw of 45mm in length by means of a cutter, and thereafter subjected to acorrosion test that one cycle of salt water spraying (aqueous solutionof 5 mass % NaCl: 35° C., relative humidity: 98%) for 2 hours drying(60° C., relative humidity: 30%) for 2 hours wetting (50° C., relativehumidity: 95%) for 2 hours is repeated 150 cycles, washed with water anddried, and then a tape peeling test on a cut flaw portion is conductedto measure a maximum peeled full width combining either side of the cutflaw portion. When the maximum peeled full width is not more than 6.0mm, the corrosion resistance can be evaluated to be good in thecomposite cycle corrosion test.

The test results are also shown in Table 6. As seen from these results,the steel sheets of Invention Examples, wherein the surface of the steelsheet after annealing is subjected to the pickling and repickling underthe conditions that the surface covering ratio of the iron-based oxideon the surface of the steel sheet after repickling is not more than 40%and the maximum thickness of the iron-based oxide is not more than 150nm, are small in the maximum peeled full width on all of the hot saltwater immersion test, salt water spray test and composite cyclecorrosion test and show the very good corrosion resistance aftercoating. Moreover, as the distribution in depth direction of O, Si, Mnand Fe is measured with GDS, it has been confirmed that in the steelsheets pickled under the conditions adequate for the invention, peaks ofSi and O do not appear and the Si-containing oxide layer is removedsufficiently.

TABLE 6-1 Surface properties Surface Pcckling condition Repicklingcondition covering Maximum Acid Treating Acid Treating ratio ofthickness of concentration Temperature time concentration Temperaturetime iron-based iron-based No (g/l) (° C.) (seconds) (g/l) (° C.)(seconds) Oxide (%) Oxide (nm) 1 Nitric acid: 40 10 — — — 72.6 214 2150 + 40 10 Hydrofluoric 40 1 39.5 158 3 Hydrochloric acid: 0.1 10 35.3157 4 acid: 15 30 30.4 162 5 40 10 Hydrofluoric 40 1 38.2 149 6 acid: 310 33.1 148 7 30 27.8 144 8 40 10 Hydrofluoric 40 1 35.2 119 9 acid: 1010 30.3 114 10 30 25.9 124 11 40 10 Hydrofluoric 40 1 30.1 91 12 acid:50 10 26.2 88 13 30 21.2 83 14 Nitric acid: 40 10 Hydrofluoric 40 1 39.7163 15 50 + acid: 0.1 10 36.1 167 16 Hydrofluoric 30 32.1 159 17 acid:50 40 10 Hydrofluoric 40 1 37.8 148 18 acid: 3 10 34.3 147 19 30 28.5149 20 40 10 Hydrofluoric 40 1 36.2 146 21 acid: 10 10 32.4 144 22 3028.3 148 23 40 10 Hydrofluoric 40 1 31.2 115 24 acid: 50 10 25.6 120 2530 22.3 119 Full width peeled after corrosion test (mm) Temperature ofphosphate treating solution: 35° C. 33° C. Hot salt water Salt waterComposite cycle No Immersion test Spray test corrosion test Remarks 16.5 5.8 8.2 8.4 Comparative example 2 5.2 4.3 6.2 6.3 Invention example3 5.2 4.1 6.1 6.3 Invention example 4 5.3 4.2 6.3 6.4 Invention example5 4.9 4.0 5.9 5.9 Invention example 6 5.0 3.9 5.8 5.9 Invention example7 4.8 4.0 5.7 5.9 Invention example 8 4.7 3.8 5.5 5.6 Invention example9 4.7 3.7 5.6 5.4 Invention example 10 4.8 3.8 5.6 5.6 Invention example11 4.4 3.6 5.1 5.2 Invention example 12 4.3 3.4 4.9 5.2 Inventionexample 13 4.2 3.1 4.6 4.4 Invention example 14 5.3 4.4 6.3 6.2Invention example 15 5.5 4.3 6.3 6.4 Invention example 16 5.4 4.4 6.26.2 Invention example 17 5.0 3.9 5.9 6.0 Invention example 18 4.8 3.95.8 6.0 Invention example 19 4.9 4.0 5.8 5.9 Invention example 20 4.94.0 5.9 5.9 Invention example 21 4.9 3.9 5.7 6.0 Invention example 224.8 3.9 5.9 6.0 Invention example 23 4.5 3.7 5.5 5.5 Invention example24 4.6 3.8 5.6 5.4 Invention example 25 4.4 3.8 5.5 5.6 Inventionexample

TABLE 6-2 Surface properties Surface Pcckling condition Repicklingcondition covering Maximum Acid Treating Acid Treating ratio ofthickness of concentration Temperature time concentration Temperaturetime iron-based iron-based No (g/l) (° C.) (seconds) (g/l) (° C.)(seconds) Oxide (%) oxide (nm) 26 Nitric acid: 40 10 Sulfuric 40 1 39.5165 27 150 + acid: 0.1 10 35.3 168 28 Hydrochloric 30 30.4 170 29 acid:15 Sulfuric 40 1 38.5 148 30 acid: 8 10 33.1 146 31 30 27.6 149 32Sulfuric 40 1 35.2 121 33 acid: 75 10 30.3 118 34 30 25.9 117 35Sulfuric 40 1 30.1 90 36 acid: 150 10 26.2 81 37 30 21.2 86 38 Nitricacid: 40 10 Sulfuric 40 1 39.7 170 39 50 + acid: 0.1 10 36.1 169 40Hydrofluoric 30 32.1 174 41 acid: 50 Sulfuric 40 1 38.7 149 42 acid: 810 33.9 148 43 30 27.4 148 44 Sulfuric 40 1 36.2 145 45 acid: 75 10 32.4148 46 30 28.3 147 47 Sulfuric 40 1 31.2 118 48 acid: 150 10 25.6 115 4930 22.3 113 Full width peeled after corrosion test (mm) Temperature ofphosphate treating solution: 35° C. 33° C. Hot salt water Salt waterComposite cycle No Immersion test Spray test corrosion test Remarks 265.4 4.4 6.2 6.4 Invention example 27 5.5 4.4 6.4 6.5 Invention example28 5.7 4.5 6.3 6.5 Invention example 29 4.9 4.0 5.8 5.9 Inventionexample 30 4.8 3.9 5.8 5.7 Invention example 31 4.8 3.8 5.7 5.9Invention example 32 4.6 3.9 5.6 5.7 Invention example 33 4.6 3.7 5.55.4 Invention example 34 4.4 3.6 5.6 5.8 Invention example 35 4.6 3.65.2 5.2 Invention example 36 4.3 3.4 5.0 4.9 Invention example 37 4.43.3 4.7 5.0 Invention example 38 5.6 4.6 6.4 6.5 Invention example 395.5 4.6 6.6 6.8 Invention example 40 5.8 4.7 6.7 6.8 Invention example41 4.8 4.0 5.9 6.0 Invention example 42 4.9 3.9 5.8 5.9 Inventionexample 43 5.0 4.0 5.8 5.9 Invention example 44 4.9 4.0 5.8 5.9Invention example 45 4.8 3.9 5.9 5.8 Invention example 46 4.8 3.9 5.76.0 Invention example 47 4.6 3.6 5.5 5.6 Invention example 48 4.4 3.75.4 5.7 Invention example 49 4.5 3.5 5.3 5.4 Invention example

TABLE 6-3 Surface properties Surface Pcckling condition Repicklingcondition covering Maximum Acid Treating Acid Treating ratio ofthickness of concentration Temperature time concentration Temperaturetime iron-based iron-based No (g/l) (° C.) (seconds) (g/l) (° C.)(seconds) Oxide (%) oxide (nm) 50 Nitric acid: 40 10 Hydrofluoric 40 135.5 152 51 150 + acid: 5 + 10 30.6 155 52 Hydrochloric Sulfuric 30 26.3154 acid: 15 acid: 5 53 Nitric acid: 40 10 Hydrofluoric 40 1 34.3 146 54150 + acid: 3 + 10 29.5 145 55 Hydrochloric Sulfuric 30 25.6 143 acid:15 acid: 8 56 Nitric acid: 40 10 Hydrofluoric 40 1 36.5 155 57 50 +acid: 5 + 10 31.6 158 58 Hydrofluoric Sulfuric 30 28.4 156 acid: 50acid: 5 59 Nitric acid: 40 10 Hydrofluoric 40 1 35.7 148 60 50 + acid:3 + 10 30.9 147 61 Hydrofluoric Sulfuric 30 27.0 148 acid: 50 acid: 8Full width peeled after corrosion test (mm) Temperature of phosphatetreating solution: 35° C. 33° C. Hot salt water Salt water Compositecycle No Immersion test Spray test corrosion test Remarks 50 5.1 4.1 6.16.2 Invention example 51 5.1 4.2 6.2 6.2 Invention example 52 5.2 4.26.3 6.4 Invention example 53 4.8 3.9 5.9 5.9 Invention example 54 4.83.8 5.9 5.7 Invention example 55 4.6 3.7 5.7 5.8 Invention example 565.2 4.2 6.2 6.2 Invention example 57 5.3 4.4 6.3 6.2 Invention example58 5.3 4.2 6.3 6.4 Invention example 59 4.9 3.8 5.9 5.9 Inventionexample 60 4.9 4.0 5.9 5.8 Invention example 61 5.0 3.9 5.8 6.0Invention example

INDUSTRIAL APPLICABILITY

The cold-rolled steel sheets produced according to the invention notonly are excellent in the corrosion resistance after coating but alsohave a high strength and a good workability, so that they can bepreferably used as not only a starting material used in members of theautomotive vehicle body but also a starting material for applicationsrequiring the same properties such as household electrical goods,building members and so on.

1. A method of producing a cold-rolled steel sheet, comprising steps ofcold rolling a steel sheet, continuously annealing, pickling and furtherrepickling it.
 2. A method of producing a cold-rolled steel sheetaccording to claim 1, wherein the repickling uses a non-oxidizable aciddifferent from an acid used in the pickling before repickling.
 3. Amethod of producing a cold-rolled steel sheet according to claim 2,wherein the non-oxidizable acid is any of hydrochloric acid, sulfuricacid, phosphoric acid, pyrophosphoric acid, formic acid, acetic acid,citric acid, hydrofluoric acid, oxalic acid and a mixed acid of two ormore thereof.
 4. A method of producing a cold-rolled steel sheetaccording to claim 2, wherein the non-oxidizable acid is any ofhydrochloric acid with a concentration of 0.1-50 g/L, sulfuric acid witha concentration of 0.1-150 g/L and a mixed acid of 0.1-20 g/L ofhydrochloric acid and 0.1-60 g/L of sulfuric acid.
 5. A method ofproducing a cold-rolled steel sheet according to claim 1, wherein therepickling is carried out at a temperature of a repickling solution of20-70° C. for 1-30 seconds.
 6. A method of producing a cold-rolled steelsheet according to claim 1, wherein the pickling is carried out with anyof nitric acid, hydrochloric acid, hydrofluoric acid, sulfuric acid anda mixed acid of two or more thereof.
 7. A method of producing acold-rolled steel sheet according to claim 1, wherein the pickling iscarried out with any of a mixed acid of nitric acid and hydrochloricacid wherein a concentration of nitric acid is more than 50 g/L but notmore than 200 g/L and a ratio (HCl/HNO₃) of hydrochloric acidconcentration to nitric acid concentration is 0.01-1.0, or a mixed acidof nitric acid and hydrofluoric acid wherein a concentration of nitricacid is more than 50 g/L but not more than 200 g/L and a ratio (HF/HNO₃)of hydrofluoric acid concentration to nitric acid concentration is0.01-1.0.
 8. A method of producing a cold-rolled steel sheet accordingto claim 1, wherein the steel sheet comprises 0.5-3.0 mass % of Si.
 9. Amethod of producing a cold-rolled steel sheet according to claim 8,wherein the steel sheet has a chemical composition comprising, inaddition to Si, C: 0.01-0.30 mass %, Mn: 1.0-7.5 mass %, P: not morethan 0.05 mass %, S: not more than 0.01 mass %, Al: not more than 0.06mass % and the remainder being Fe and inevitable impurities.
 10. Amethod of producing a cold-rolled steel sheet according to claim 8,wherein the steel sheet contains, in addition to the chemicalcomposition, one or more selected from Nb: not more than 0.3 mass %, Ti:not more than 0.3 mass %, V: not more than 0.3 mass %, Mo: not more than0.3 mass %, Cr: not more than 0.5 mass %, B: not more than 0.006 mass %and N: not more than 0.008 mass %.
 11. A method of producing acold-rolled steel sheet according to claim 8, wherein the steel sheetcontains, in addition to the chemical composition, one or more selectedfrom Ni: not more than 2.0 mass %, Cu: not more than 2.0 mass %, Ca: notmore than 0.1 mass % and REM: not more than 0.1 mass %.
 12. Acold-rolled steel sheet produced by a method as claimed in claim 1,characterized in that a Si-containing oxide layer is removed from thesurface of the steel sheet by pickling after continuous annealing and asurface covering ratio of an iron-based oxide existing on the surface ofthe steel sheet after repickling is not more than 40%.
 13. A cold-rolledsteel sheet according to claim 12, wherein a maximum thickness of theiron-based oxide existing on the surface of the steel sheet afterrepickling is not more than 150 nm.
 14. A member for automobilecharacterized by using a cold-rolled steel sheet as claimed in claim
 1213. 15. A method of producing a cold-rolled steel sheet according toclaim 2, wherein the repickling is carried out at a temperature of arepickling solution of 20-70° C. for 1-30 seconds.
 16. A method ofproducing a cold-rolled steel sheet according to claim 3, wherein therepickling is carried out at a temperature of a repickling solution of20-70° C. for 1-30 seconds.
 17. A method of producing a cold-rolledsteel sheet according to claim 4, wherein the repickling is carried outat a temperature of a repickling solution of 20-70° C. for 1-30 seconds.18. A method of producing a cold-rolled steel sheet according to claim2, wherein the pickling is carried out with any of nitric acid,hydrochloric acid, hydrofluoric acid, sulfuric acid and a mixed acid oftwo or more thereof.
 19. A method of producing a cold-rolled steel sheetaccording to claim 3, wherein the pickling is carried out with any ofnitric acid, hydrochloric acid, hydrofluoric acid, sulfuric acid and amixed acid of two or more thereof.
 20. A method of producing acold-rolled steel sheet according to claim 4, wherein the pickling iscarried out with any of nitric acid, hydrochloric acid, hydrofluoricacid, sulfuric acid and a mixed acid of two or more thereof.